1. Field of the Invention
This invention relates to a method for removing cholesterol and fat from egg yolk by chelation and to a reduced-cholesterol egg product. More specifically, this invention relates to a method for removing cholesterol from egg yolk by separating lipoprotein-containing complexes from egg yolk and extracting cholesterol and fats from the complexes to form protein-containing complexes, which are incorporated into a reduced cholesterol liquid egg product.
2. Description of Related Art
Egg, especially chicken egg, is a popular and desirable component of the human diet. In particular, the nutritive quality of the protein content of egg is valued. Egg is organoleptically pleasing and useful in various manners and methods of food preparation.
Liquid whole egg consists, on the average, of about 64 percent white and about 36 percent yolk. The white contains approximately 12 percent solid matter of a primarily proteinaceous nature. The yolk contains about 50 percent solids, of which a major portion is fat and a lesser portion is protein, the fat and protein being present in approximately a 2:1 ratio. A large proportion of the fat appears to be bound to the protein in the form of lipoprotein complexes. Some of this lipid is easily extractable with fat solvents, such as hexane, and this portion of the lipid is conventionally called "free lipid." It is also referred to as "free fat." The remaining lipid typically is referred to as "bound fat." It can be extracted, but only under more rigorous conditions. The term "fat" is used here, in connection with egg yolk, to refer to both kinds of fat, in a generic sense.
Egg white or egg albumen is essentially an aqueous solution of proteins containing small amounts of other materials such as minerals and sugars, and only a trace of fat. The egg yolk, on the other hand, contains virtually all of the fat and cholesterol of whole eggs. These latter components are present in combination with egg yolk proteins as complex lipoprotein compounds. Consequently, many of the functional properties and nutritional aspects of the egg reside specifically in the yolk portion. This is evidenced by the incorporation of yolk alone as an ingredient in many basic food formulations.
Eggs contain a significant quantity of cholesterol and there is evidence that cholesterol from eggs raises human serum cholesterol level. Medical evidence relates high serum cholesterol levels to disease of the vascular system. Thus, despite the organoleptic appeal and nutritive value of eggs, consumers are becoming reluctant to eat eggs in any form.
Various methods have been proposed for removing cholesterol from eggs or egg yolks to produce a low-cholesterol egg product. However, typically these methods significantly alter the functional properties of eggs thus treated, destroying the suitability of the egg for use in traditional cooking processes and making the low-cholesterol product organoleptically unsatisfactory.
Some methods produce a solid product. One such method is disclosed in U.S. Pat. No. 3,563,765, wherein "free fat" is extracted from dry egg yolk solids by mixing the solids with solvent and heating the mixture to a temperature below 160.degree. F., preferably below 120.degree. F., to avoid denaturation of the protein. A non-polar solvent such as hexane extracts non-polar lipids and cholesterol preferentially from yolk lipoproteins. This results in partial removal of yolk fat. The extracted solids must be heated to remove, by distillation, any solvent which remains in the egg solids.
Methods of replacing fat thus extracted typically incorporate addition of unsaturated fats so as to improve the ratio of polyunsaturated fats to saturated fats in egg products produced. For example, U.S. Pat. No. 3,594,183 discloses a method for replacing the fats in a solid, low-cholesterol, egg-containing product by combining low-cholesterol egg yolk with unsaturated oil. The patent also discloses a method for preparing an egg food product comprising the re-fatted yolk solids.
Methods for producing low-cholesterol solids from liquid egg also are known. For example, U.S. Pat. No. 3,881,034 discloses that cholesterol and free fat can be removed from liquid whole egg by mixing egg with water and an organic solvent. The solvent is selected so that, during distillation separation of the solvent and fat dissolved therein form the egg product, the solvent forms an azeotrope with water at a temperature less than about 140.degree. F. Thus, the free fat and cholesterol are extracted from the egg fluid into the solvent, and the solvent and water are distilled off to leave dry egg powder which is free of cholesterol and of free fat.
Many practitioners find such solvent extraction objectionable because the extraction requires use of organic solvents, which must be separated from the egg product at a temperature less than about 140.degree.-160.degree. F. to avoid thermal denaturation of the egg proteins. Many solvents denature egg protein when extracting lipids from lipoprotein complexes. Denaturation of the egg protein degrades the organoleptic properties and qualities of the resultant product. The solvent also must be suitable for contact with food stuffs. Small quantities of solvent may remain in the egg product, even though the most stringent measures have been taken to remove the solvent. Remaining solvent is objectionable to many consumers, not only because of the health risks solvents are perceived to present, but also because solvents degrade the organoleptic properties of the product.
A method said to remove cholesterol from dried egg powder (whole egg or yolk) without leaving solvent residue in the egg product is disclosed in U.S. Pat. No. 4,104,286. Dry egg powder is contacted with an aqueous ethanol solution (typically 95 percent ethanol) which extracts free fats and cholesterol from the egg powder. The low-cholesterol, low-fat powder is separated from the aqueous solution, which now contains fats and cholesterol. The aqueous solution is further treated to saponify fatty acids, and cholesterol and saponified fatty acids are separately recovered.
Vegetable oil has been utilized to extract cholesterol from egg. For example, U.S. Pat. No. 3,717,474 discloses that cholesterol can be extracted from liquid egg yolk simultaneously with addition of unsaturated fats thereto by using high shear agitation to emulsify and ensure contact between the liquid egg yolk and the unsaturated oil. The patent discloses that high energy mixing is required throughout the treatment to disrupt the water barrier surrounding the yolk fat globules which contain the saturated fat and cholesterol to be extracted. Disruption of the barrier affords the opportunity for the yolk fat droplets and the solvent to come into intimate contact. The period of high shear agitation, which is necessary to ensure that the barrier is disrupted, must be shorter than the time required for the heat generated by the agitation to raise the temperature of the mixture being agitated to the temperature at which egg protein becomes denatured.
In the method described in U.S. Pat. No. 4,333,959, the pH of liquid egg yolk is reduced to a value not below three, preferably between about four and six, to both destabilize and reduce the viscosity of the yolk. Then, the destabilized emulsion is treated with edible oil to form a fine dispersion and extract fat and cholesterol from the yolk droplets. The dispersion is formed by homogenization in an agitator mill utilizing the microballs, in a high pressure homogenizer, or by intense vibration such as that used in continuous disintegrators. The dispersion then is centrifuged to separate low-cholesterol egg yolk from the oil extractant. The patent further discloses that extraction under the same conditions of shear without reduction of the pH of the yolk produces lower, albeit significant, cholesterol reduction in the yolk.
These methods, although they do not require an organic solvent which is not an edible oil, are not entirely satisfactory, because intense agitation is required to destabilize the yolk and maintain the dispersion of yolk in the oil.
Methods of removing cholesterol together with other egg components, such as phospholipids and lipoprotein complexes (which contain the "bound" fat), often are utilized to perform qualitative and quantitative analysis on individual components or to obtain purified components. For example, the method disclosed in U.S. Pat. No. 4,104,286 and described above further comprises recovery of pure cholesterol from the ethanol extractant.
Because the phospholipid concentration of chicken egg yolks is about 8 times the cholesterol concentration, such yolks often are utilized as a source of phospholipids. Yolk phospholipid can be recovered from egg yolk by zinc chloride precipitation of yolk lipids extracted by ethanol, or by other extraction processes known to skilled practitioners.
Typically, these phospholipid and lipoprotein complex extraction processes involve use of methanol and ethanol, which achieve complete lipid removal by denaturing the lipoprotein complexes. Thus-released lipids then are recovered from the extractant. The remaining egg product is not organoleptically satisfying, however, even when a portion of the lipids is replaced. Thus, such processes are not satisfactory for producing organoleptically pleasing low cholesterol egg products. Not only must an organic solvent which is not an edible oil be utilized, but also the texture of the yolk is unacceptable after denaturation of the lipoproteins.
A method for separating phospholipids from egg yolk so that the proteins are not denatured is described in Hatta, "Separation of Phospholipids from Egg Yolk and Recovery of Water-soluble Proteins," J. Food Sci. 53:425-427 (1988). In Hatta's method, sodium alginate aqueous solution is added to dilute egg yolk, the pH preferably is adjusted to between about 5.8 and 6.4, and the mixture is held at room temperature for about 30 minutes to form a precipitate comprising sodium alginate and lipoprotein. The aqueous fraction containing water-soluble protein is separated from the alginate/lipoprotein precipitate by centrifugation at 10,000 G for about 15 minutes. Thus, sodium alginate solution was utilized to separate water-soluble proteins from egg yolk, minimizing the lipid content and maximizing the protein content of the supernatant to avoid denaturation of the water-soluble protein.
The alginate/lipoprotein precipitate is extracted with ethanol to prepare a crude phospholipid extract. Crude extract is further separated, sometimes by centrifugation, into various fractions which are purified. For example, the crude phospholipid extract is treated with zinc chloride in ethanol to form purified phospholipid extract, which then is washed with acetone.